Cold Rolled Steel Wire, Method and Reinforcement of Flexible Conduits

ABSTRACT

A cold rolled steel wire having the following chemical composition expressed in percent by weight, 0.2≦C %≦0.6, 0.5≦Mn %≦1.0, 0.1≦Si≦0.5%, 0.2≦Cr≦1.0%, P≦0.020%, S≦0.015%, N≦0.010%, and optionally not more than 0.07% Al, not more than 0.2% Ni, not more than 0.1% Mo and not more than 0.1% Cu, the balance being iron and the unavoidable impurities due to processing. This wire has a microstructure including bainite and, optionally, up to 35% acicular ferrite and up to 15% pearlite. A fabrication method and flexible conduits for hydrocarbon extraction are also provided.

The present invention relates to the field of metallurgy as applied tomaritime oil extraction. It relates in particular to steel wires thatcan be used as reinforcing or structural elements of components orstructures submerged in deep water, such as flexible offshore conduits,whether they are used to transport liquid or gas hydrocarbons.

BACKGROUND

It is known that a primary requirement relating to wires of this type,in addition to excellent mechanical characteristics, is good resistanceto hydrogen embrittlement in an acid environment where sulfur ispresent, in particular in the form of H₂S which is present in the fluidsand hydrocarbons being transported.

The steel wires currently commercially available for offshoreutilization are principally made of the low-alloy grades that have atensile strength Rm of approximately 800 MPa.

To fabricate these cold rolled wires in the known manner, manganesesteels comprising from 0.15 to 0.80% by weight carbon are used, theinitial microstructure of which is ferritic-pearlitic. After shaping ofthe initial round rolled wire rod, an appropriate stress-relief heattreatment is applied to achieve the required hardness. However, the coldrolled wires obtained by these conventional processes cannot withstandthe relatively severe acidity encountered in deep waters, due to thestrong presence of H₂S in the hydrocarbons transported.

In addition, flexible offshore conduits must currently be appropriatefor use at increasingly greater underwater depths, which requires anincrease in the mechanical breaking strength beyond 800 MPa as well ascorrosion-fatigue strength enabling them to resist corrosion generatedby the presence of H₂S and CO₂.

In addition, market constraints regarding prices are becomingincreasingly tight, which has an adverse effect on the customary use ofnoble alloy elements such as chromium, niobium etc. or long or multipleand therefore expensive processing steps, above all if they must becarried out under hot conditions.

BRIEF SUMMARY

An object of the invention is to make available a steel wire that hasvery good properties of resistance to hydrogen embrittlement in an acidenvironment (of the H₂S type) and very good corrosion-fatigue strengthproperties (CO₂+H₂S) to satisfy the new constraints in the oil and gasmarkets, in particular by employing a low-alloy steel composition.

More particularly, an objective is the absence of internal cracks aftertests under stress for 30 days at a pH of 4.1 in an environmentcontaining 5 mbar H₂S or even more (resistance to hydrogenembrittlement).

An additional objective is the absence of fatigue failure after 2million bending cycles under an alternating stress of +100 MPa to +600MPa in a corrosive environment containing CO₂ and H₂S. Even moreparticularly and preferably, an objective is the absence of fatiguefailure even up to 4 million cycles (corrosion-fatigue strength).

In addition to these usage properties of the wire, an additionalobjective is excellent mechanical properties and, in particular,mechanical breaking strength from 800 to 1300 MPa and a ductility Agreater than or equal to 10% to facilitate the shaping operations.

The present invention provides a cold rolled steel wire, having thefollowing chemical composition, expressed in percent by weight:

0.2≦C %≦0.6;

0.5≦Mn %≦1.0;

0.1≦Si≦0.5%;

0.2≦Cr≦1.0%;

P≦0.020%;

S≦0.015%;

N≦0.010%;

and optionally not more than 0.07% Al, not more than 0.2% Ni, not morethan 0.1% Mo and not more than 0.1% Cu, the balance being iron and theunavoidable impurities due to processing, wherein this wire has amicrostructure comprising bainite and, optionally, up to 35% acicularferrite and up to 15% pearlite.

The present invention also provides a fabrication method for a coldrolled wire comprising the steps of:

procuring a hot rolled steel wire rod, the diameter of which is greaterthan or equal to 10 mm,

subjecting this wire rod to an austenitization heat treatment at atemperature greater than 950° C., then

cooling this wire rod by an isothermal quenching to a temperaturebetween 350 and 600° C. to obtain a structure comprising at least 50%bainite and, optionally, up to 35% acicular ferrite and up to 15%pearlite, then finishing the cooling to ambient temperature at a coolingrate between 30° C./s and 100° C./s,

subjecting this cooled wire rod to a cold mechanical transformationoperation conducted with a total rate of work hardening between 40 and90% to obtain a mechanical tensile strength Rm of at least 800 MPa, then

subjecting it to an optional stress relief heat treatment.

The present invention further provides a flexible conduit for thehydrocarbon extraction sector comprising at least one cold rolled wireor obtained by a fabrication method.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will be elucidated withreference to the followinq drawings, in which:

FIG. 1 is a micrographic image showing the different constituents of themicrostructure obtained with grade 1 (bainite, acicular ferrite andlamellar pearlite) and that comprises less than 35% acicular ferrite andless than 15% pearlite; and

FIG. 2 is a micrographic image showing the different constituents of themicrostructure obtained with grade 3 comprising 70% bainite and 30%acicular ferrite and lamellar pearlite.

DETAILED DESCRIPTION

According to the present invention, the grade of steel used for thewires according to the invention comprises the following elements inpercent by weight:

0.2≦C %≦0.6

0.5≦Mn %≦1.0

0.1≦Si≦0.5%

0.2≦Cr≦1.0%

P≦0.020%

S≦0.015%

N≦0.010%

and optionally not more than 0.07% Al, not more than 0.2% Ni, not morethan 0.1% Mo and not more than 0.1% Cu.

The carbon content is between 0.2 and 0.6 wt. %. The low value makes itpossible to guarantee the achievement of sufficient hardness at the exitfrom the heat treatment. The maximum content is limited to 0.6% tosufficiently protect the ductility necessary for cold-forming during thefabrication of the wire.

The manganese content is between 0.5 and 1.0 wt. %. The low value makesit possible to guarantee the achievement of the desired majority bainitemicrostructure. The maximum content is limited to 1.0% to prevent theformation of relatively non-deformable phases such as martensite.

The silicon content is between 0.1 and 0.5 wt. %. The low value, incombination with the low chromium level, makes it possible to guaranteethe achievement of the microstructure specified by the invention in theentire cross section of the shaped wire. The maximum content is limitedto 0.5%, here again to prevent the formation of martensite during theheat treatment, because this phase is subsequently not deformable.

The chromium content is between 0.2 and 1.0 wt. %. The low value, incombination with the low level of silicon, makes it possible toguarantee the achievement of the microstructure specified by theinvention in the entire cross section of the shaped wire. The maximumcontent is limited to 1.0% to prevent the formation of martensite duringthe heat treatment.

The phosphorus content is limited to 0.020 wt. % to limit segregations,which have an adverse effect on the hydrogen embrittlement resistanceand on fatigue behavior.

The sulfur content is limited to 0.015 wt. % to limit the presence ofinclusions, which have an adverse effect on fatigue behavior andhydrogen embrittlement.

The nitrogen content is limited to 0.010 wt. % to limit the presence ofnitrides, which have an adverse effect on fatigue behavior.

The grade can also comprise up to 0.07% Al, up to 0.2% Ni, up to 0.1% Moand up to 0.1% Cu. The presence of these elements is limited becausethey are the cause of dispersive behaviors during the heat treatment.

Another essential element of the wire according to the present inventionis that the microstructure of the steel of which they are made includesbainite. It can also include acicular ferrite up to an area percentageof 35%. Generally lamellar pearlite can also be present up to an areapercentage of 15%, preferably up to an area percentage of 10% and moreparticularly preferably is completely absent from the microstructurebecause its presence is not desirable.

The desired microstructure can therefore be bainitic orbainitic-ferritic, the sum of the bainite and acicular ferrite phasesbeing preferably greater than an area percentage of 50%, preferablygreater than 70% and most particularly preferably greater than 90%, oreven equal to an area percentage of 100%.

Besides pearlite, the proportion of which is limited, it is alsodesirable to avoid the presence of martensite, which would make itimpossible to fabricate a wire according to the present invention.

The cold rolled wires according to the present invention (sometimescalled shaped wire) may assume any cross section appropriate to theirfinal utilization. In particular, they may have a straight round crosssection, which is generally obtained, for example, by simple drawingfrom a larger-diameter wire rod.

They can also have a cross section that is rectangular or flat orU-shaped, Z-shaped, T-shaped etc., which will generally require thecombination of a drawing operation and a rolling operation. These lattercross sections with a more complex shape will make it possible for thewires to be fitted into one another on the edges or to be fastenedtogether to form articulated bracing bundles.

They are more particularly intended for offshore oil operations toconstitute the reinforcing wire, tie wire or arch wires that go into thestructure of pipelines and other flexible conduits. The cold rolledsteel wires go around inside the pipelines between two layers ofextruded polymers in a so-called “annular” zone.

The wires according to the present invention can be fabricated by anysuitable process that makes it possible to obtain the desired usagecharacteristics. However, it is preferable to use the method accordingto the present invention, which includes the steps comprising of:

procuring a hot rolled steel wire rod, the diameter of which is greaterthan or equal to 10 mm,

subjecting this wire rod to an austenitization heat treatment at atemperature greater than 950° C., then

cooling this wire rod by an isothermal quenching to a temperaturebetween 350 and 600° C. to obtain a structure comprising at least 50%bainite and, optionally, up to 35% acicular ferrite and up to 15%pearlite, then finishing the cooling to ambient temperature at a coolingrate between 30° C./s and 100° C./s,

subjecting this cooled wire rod to a cold mechanical transformationoperation conducted with a total rate of work hardening between 40 and90% to obtain a mechanical tensile strength Rm of at least 800 MPa, then

subjecting it to an optional stress relief heat treatment.

The method according to the present invention can also comprise thefollowing characteristics, considered individually or in combination:

the austenitization heat treatment undergone by the wire rod isperformed continuously, preferably in a gas furnace or in an inductionfurnace, on the previously unspooled wire,

the austenitization heat treatment is performed for a length of timebetween 2 and 10 minutes,

the wire rod is cooled by isothermal quenching in a molten, preferablylead- or salts-based (mixture of sodium or potassium nitrates/nitrites),bath or in a fluidized bed of the same elements, the immersion timebeing preferably between 1 and 10 minutes, preferably followed by awater cooling,

the cold mechanical transformation operation comprises a drawing stepfollowed by a cold rolling step, the drawing being preferably performedto obtain a cross section reduction of at least 15 or even 20%, and thecold rolling is preferably performed to obtain a thickness reduction ofat least 30%,

the cold mechanical transformation operation is performed to obtain amechanical tensile strength Rm of at least 1000 MPa and is followed by astress relief heat treatment performed so as not to reduce themechanical tensile strength Rm to below 800, or even 850 or 900 MPa.

A flexible conduit for the hydrocarbon extraction sector is furtherprovided. The conduit includes at least one cold rolled wire accordingto a preferred embodiment of the present invention or obtained by thefabrication method of a preferred embodiment of the present invention.

The stress relief operation makes it possible, among other things, toimprove the resistance of the wire to hydrogen embrittlement.

To better explain the present invention, tests were conducted, althoughthe tests are merely for purposes of illustration and are not intendedto be restrictive.

Tests

The tests conducted to evaluate the performance of the wire according tothe present invention were conducted under the following conditions:Hydrogen embrittlement (HIC & SSCC test—NACE TM0177 and NACE TM0284standards)

The steel wire was immersed in an aqueous solution having a pH of 4.1,through which a gas containing CO₂ and 5 mbar of H₂S was bubbled, undera stress of 650 MPa. The test was conducted over 30 days, at theconclusion of which the wire was examined using ultrasound to check forthe potential presence of internal cracks.

Fatigue Strength in a Corrosive Environment

The steel wire was immersed in an aqueous solution having a pH of 5,through which a gas containing CO₂ and up to 5 mbar of H₂S was bubbled,under an alternating bending stress of 100 MPa to 500 MPa. The test wasstopped when the wire broke and the number of millions of cycles reachedwas recorded.

First a series of grades was prepared, the chemical composition ofwhich, in wt. %, is listed in Table 1:

TABLE 1 C Mn Si Cr P S N Al Ni Mo Cu 1 0.360 0.680 0.210 0.250 0.0100.011 0.007 0.045 0.050 0.010 0.050 2 0.340 0.700 0.180 0.350 0.0110.008 0.006 0.043 0.060 0.015 0.060 3 0.320 0.725 0.180 0.406 0.0110.010 0.003 0.020 0.020 0.015 0.009

For grades 1 to 3, hot rolled round wire rods 15 mm in diameter werethen conventionally fabricated, which were coiled in a spool. In asecond step, the previously unspooled wire was then austenitized at1000° C. in a gas furnace for 6 minutes.

An isothermal quenching was then carried out in a bath of molten leadfor 5 minutes at 500° C. for grades 1 and 2 and at 410° C. for grade 3,followed by a water cooling to give the wire a ferrite/bainitestructure. FIG. 1 is a micrographic image showing the differentconstituents of the microstructure obtained with grade 1 (bainite,acicular ferrite and lamellar pearlite) and that comprises less than 35%acicular ferrite and less than 15% pearlite. FIG. 2 is a micrographicimage showing the different constituents of the microstructure obtainedwith grade 3 comprising 70% bainite and 30% acicular ferrite andlamellar pearlite.

The wires were then drawn with a cross section reduction of 30% toobtain a round wire with a diameter of 12.54 mm, then cold rolled with athickness reduction of 50% to obtain a flat wire, having a cross sectionof 16 mm×6.3 mm. The total work hardening rate that corresponds to thepercentage of cross section reduction at the conclusion of the two coldoperations here was 57%. The wires were then subjected to a stressrelief annealing at a temperature of 600° C. for 30 seconds.

They were then subjected to tests to evaluate their usage properties,the results of which are presented in Table 2:

TABLE 2 Rm A Number of internal Millions of cycles before Grade (MPa)(%) cracks fracture 1 900 13 0 2,000,000 (650 MPa) 2 1000 11 0 5,000,000(650 MPa) 3 1200 11 0 1,800,000 (650 MPa) NR: not performed

The invention is not limited to the examples described above, butextends to multiple variants and equivalents.

What is claimed is: 1-12. (canceled) 13: A cold rolled steel wire,having a chemical composition comprising, expressed in percent byweight:0.2≦C %≦0.60.5≦Mn %≦1.00.1≦Si≦0.5%0.2≦Cr≦1.0%P≦0.020%S≦0.015%N≦0.010%, the balance being iron and the unavoidable impurities due toprocessing, wherein this wire has a microstructure comprising bainiteand, optionally, up to 35% acicular ferrite and up to 15% pearlite. 14:The cold rolled steel wire according to claim 13, further comprising≦0.07% Al. 15: The cold rolled steel wire according to claim 13, furthercomprising ≦0.2% Ni. 16: The cold rolled steel wire according to claim13, further comprising ≦0.1% Mo. 17: The cold rolled steel wireaccording to claim 13, further comprising ≦0.1% Cu. 18: The cold rolledwire according to claim 13, wherein the cold rolled wire exhibits nointernal cracking after 30 days of exposure to an environment containingH₂S at a pH of 4.1. 19: The cold rolled wire according to claim 13,wherein the cold rolled wire exhibits no breakage before 2 millionbending cycles under an alternating stress of +100 to +500 MPa in anenvironment containing H₂S and CO₂. 20: The cold rolled wire accordingto claim 13, wherein the cold rolled wire has a mechanical tensilestrength Rm is from 900 to 1300 MPa. 21: The cold rolled wire accordingto claim 13, wherein the cold rolled wire has a ductility A greater thanor equal to 10%. 22: A fabrication method for a cold rolled wireaccording to claim 13 comprising the steps of: procuring a hot rolledsteel wire rod, the diameter of which is greater than or equal to 10 mm;subjecting the wire rod to an austenitization heat treatment at atemperature greater than 950° C.; then cooling the wire rod by anisothermal quenching to a temperature that is from 350 to 600° C. toobtain a structure comprising at least 50% bainite and, then finishingthe cooling to ambient temperature at a cooling rate that is from 30°C./s to 100° C./s; and subjecting the cooled wire rod to a coldmechanical transformation operation conducted with a total rate of workhardening from 40 to 90% to obtain a mechanical tensile strength Rm ofat least 800 MPa. 23: The fabrication method according to claim 22,further comprising the step of: subjecting the wired rod to a stressrelief heat treatment. 24: The fabrication method according to claim 22,wherein the structure includes up to 35% acicular ferrite or up to 15%pearlite. 25: The fabrication method according to claim 22, wherein theaustenitization heat treatment undergone by the wire rod is performedcontinuously in a gas furnace or in an induction furnace on a previouslyunspooled wire. 26: The fabrication method according to claim 22,wherein the austenitization heat treatment is performed for a length oftime from 2 to 10 minutes. 27: The fabrication method according to claim22, wherein the wire rod is cooled by isothermal quenching in a moltenlead- or salts-based bath and the immersion time is from 1 to 10minutes, followed by a water cooling. 28: The fabrication methodaccording to claim 22, wherein the cold mechanical transformationoperation includes a drawing step followed by a cold rolling step, thedrawing step resulting in a reduction of a cross section by at least 15%and the cold rolling step resulting in a reduction of a thickness by atleast 30%. 29: The fabrication method according to claim 22, wherein thecold mechanical transformation operation results in a mechanical tensilestrength Rm of at least 1000 MPa and is followed by a stress relief heattreatment carried out so as not to reduce the mechanical tensilestrength Rm to less than 800 MPa. 30: A flexible conduit for thehydrocarbon extraction sector comprising: at least one cold rolled wireaccording to claim
 13. 31: A flexible conduit for the hydrocarbonextraction sector comprising: at least one cold rolled wire obtained bythe method according to claim
 22. 32: A fabrication method for a coldrolled wire according to claim 13 consisting of the following steps:procuring a hot rolled steel wire rod, the diameter of which is greaterthan or equal to 10 mm; subjecting the wire rod to an austenitizationheat treatment at a temperature greater than 950° C.; then cooling thewire rod by an isothermal quenching to a temperature that is from 350 to600° C. to obtain a structure comprising at least 50% bainite andoptionally up to 35% acicular ferrite and up to 15% pearlite, thenfinishing the cooling to ambient temperature at a cooling rate that isfrom 30° C./s to 100° C./s; and subjecting the cooled wire rod to a coldmechanical transformation operation conducted with a total rate of workhardening from 40 to 90% to obtain a mechanical tensile strength Rm ofat least 800 MPa; and optionally subjecting the wired rod to a stressrelief heat treatment.